The need for improved medical examination and surgical lamps is driven by surgeon and operating room nurse preferences for superior illumination during modern surgical procedures, which frequently take many hours. Examples of such time-intensive procedures are limb reattachments in post trauma situations, open heart surgery and organ transplants. Shadow-free illumination of deep body cavities is required to eliminate eye strain and fatigue of the operating staff. It is also important to minimize downtime of the lamps and to simplify maintenance.
Surgeons respond in part to color characteristics of the body parts being observed. However, perceived color is influenced by the illuminating light. There is therefore a need for surgical lamps having high color rendering values. Also, surgeons must look closely at small body parts and into narrow cavities. There is therefore a need for a high level of illumination. For similar reasons, there is a need for lamps with acceptable color temperature, which can be moved and directed at will (universal burn position) and which provide shadow-free illumination of the operating zone. Long lamp life is also important.
The light source is commonly focused on the patient, thereby heating the operating area. Prolonged or high level heating of the patient can be injurious. There is therefore a need for a surgical lamp that minimizes temperature rise in the operating area, while delivering superior illumination.
A surgical lighting system has a stringent set of requirements. It must provide a high light level to the operating area with a spectral distribution and intensity that both supports the surgeon in his or her task, yet is not detrimental to the patient. There should be no dark shadows in the operating area, and the patient's tissue, organs and blood should be illuminated with the correct color. The perception of the smallest tissue features during the surgical procedure can be important. Sometimes the surgeon wants to see into deep body cavities, so light should come from many directions. Tissue desiccation can become an issue as body tissues exposed during surgery rapidly lose moisture. Consequently, the patient must not be excessively radiated with infrared energy which would dry the tissue. The radiant energy in the spectrum between 800 and 1000 nanometers should be kept to a minimum, as this is a spectral band of absorption by tissue and water, and contributes nothing to visual perception. Yet this spectral band is present in almost all conventional sources.
As shown in FIG. 8, the light from the surgical illuminator, for general surgery, should have color coordinates that fall within an area described by a five-sided polygon 400 on the 1931 CIE Chromaticity Diagram. Correlated color temperatures within this polygon range from 3500K to 6700K, but the color temperature of the surgical illuminator is nominally preferred to be at 4500K. A color rendering index (CRI) greater than 85 and preferably greater than 90 is required for this light source. In addition, the specific saturated red color rendering index (R9), which is not included in the computation of general CRI should be high, for example, above 60.
Surgical light sources should be flicker-free and have the ability to maintain their color properties for any lamp position. These requirements have been the major impediments to the introduction of electroded metal halide lamps into the surgical lighting area. Surgical illumination requires instant hot restart or operation of a backup illumination system following a short power interruption. Lamp life should be in excess of 1000 hours. Tungsten halogen lamps used in critical surgical applications usually undergo periodic preventive replacement. Surgical lamps must also be explosion-proof and free of electromagnetic interference (EMI), as the lamps operate in close proximity to explosive gases and highly sensitive electronic monitoring equipment.
In prior art surgical illuminators, a light source is placed inside a large area polygon reflector to direct light to the operating area from as large a spatial angle as possible. This has the advantage of reducing shadowing in the operating area by the surgeon's head and shoulders. Typically, a tungsten halogen lamp is used. Significant light filtering is necessary to eliminate the sizable component of infrared radiation generated by a tungsten halogen lamp. The infrared light filter also color corrects the tungsten halogen lamp by suppressing some red radiation to produce a higher color temperature. The normal color correction of tungsten halogen lamps then has a tendency to reduce the saturated red, or R9, index, which can affect viewing.
Electrodeless high intensity discharge (EHID) lamps have been described extensively in the prior art. In general, EHID lamps include an electrodeless lamp capsule containing a volatilible fill material and a starting gas. The lamp capsule is mounted in a fixture which is designed for coupling high frequency power to the lamp capsule. The high frequency power produces a light-emitting plasma discharge within the lamp capsule. Recent advances in the application of high frequency power to lamp capsules operating in the tens of watts range are disclosed in U.S. Pat. No. 5,070,277, issued Dec. 3, 1991, to Lapatovich; U.S. Pat. No. 5,113,121, issued May 12, 1992, to Lapatovich, et al.; U.S. Pat. No. 5,130,612, issued Jul. 14, 1992, to Lapatovich et al.; U.S. Pat. No. 5,144,206, issued Sep. 1, 1992, to Butler et al.; and U.S. Pat. No. 5,241,246, issued Aug. 31, 1993, to Lapatovich, et al. As a result, compact EHID lamps and associated applicators have become practical.
The above patents disclose small, cylindrical lamp capsules wherein high frequency energy is coupled to opposite ends of the lamp capsule with a 180.degree. phase shift. The applied electric field is generally colinear with the axis of the lamp capsule and produces a substantially linear discharge within the lamp capsule. The fixture for coupling high frequency energy to the lamp capsule typically includes a planar transmission line, such as a microstrip transmission line, with electric field applicators, such as helices, cups or loops, positioned at opposite ends of the lamp capsule. The microstrip transmission line couples high frequency power to the electric field applicators with a 180.degree. phase shift. The lamp capsule is typically positioned in a gap in the substrate of the microstrip transmission line and is spaced above the plane of the substrate by a few millimeters, so that the axis of the lamp capsule is colinear with the axes of the field applicators.
Electrodeless high intensity discharge lamps for use in automotive illumination systems are disclosed in the aforementioned U.S. Pat. Nos. 5,070,277 and 5,113,121 and in U.S. Pat. No. 5,299,100 issued Mar. 29, 1994 to Bellows et al. These systems require good light quality, reliability and long life, but are not required to provide exceptional color rendering. Consequently, the use of sodium scandium chemistry is common in EHID automotive headlamps, with general color rendering indexes of about 60-70. Thus, prior art EHID lamps have not met the requirements discussed above for surgical illumination.
Electrodeless lamps are also disclosed in U.S. Pat. No. 5,508,592 issued Apr. 16, 1996 to Lapatovich et al; U.S. Pat. No. 5,498,937 issued Mar. 12, 1996 to Korber et al; U.S. Pat. No. 5,498,928 issued Mar. 12, 1996 to Lapatovich et al; U.S. Pat. No. 5,471,109 issued Nov. 28, 1995 to Gore et al; U.S. Pat. No. 5,448,135 issued Sep. 5, 1995 to Simpson; U.S. Pat. No. 5,359,264 issued Oct. 25, 1994 to Butler et al; U.S. Pat. No. 5,339,008 issued Aug. 16, 1994 to Lapatovich et al; and U.S. Pat. No. 5,280,217 issued Jan. 18, 1994 to Lapatovich et al.